1. Field of the Invention
This invention relates to an ophthalmic measurement apparatus. More particularly, this invention relates to an ophthalmic measurement apparatus that irradiates the interior of a subject's eye with a beam of laser light and uses the light scattered from the interior of the eye to perform specific measurements.
2. Description of the Prior Art
Measurement of floating cells in the anterior chamber of the eye is of critical importance when diagnosing ophthalmic inflammations, especially malfunctions of the blood-aqueous barrier and uveitis. Conventionally a slit lamp microscope is often used for this, with grading being determined via the naked eye. On the other hand, a photographic measuring method has been developed to provide quantitative measurements. However, no method has yet been perfected that is readily applicable to clinical examinations.
An example of an ophthalmic measurement method in which the eye is irradiated with a beam of laser light and the light scattered from the eye is detected is disclosed in the Japanese Patent Publication No. Sho 64-2623 (corresponding to U.S. Pat. No. 5,000,562).
However, in such methods the size of the mask aperture is fixed and the concentration of floating cells in the anterior chamber (hereinafter referred to as "cells") may be such that two or more cells enter the laser beam at the same time. This can be a problem if two cells are counted as one because the indicated count will be lower than the actual count, degrading the measurement accuracy. Reducing the size of the mask aperture to prevent this also decreases the volume of the measurement portion, and in cases where the cell concentrations are low this results in an intermittent count that degrades reproducibility and produces inconsistent results.
This problem, will now be explained with reference to FIGS. 9a and 9b. FIG. 9a shows the measurement field of view in an eye as observed using a measurement system apparatus. In the drawing, L indicates a laser beam. Measurements by the system's photosensing section are performed laterally on the region illuminated by the laser beam. The measurement field of view is defined by the use of a specific aperture of a mask arranged in front of the system photosensor.
The presence of two cells (particles) C in the laser beam that are in line along the direction of beam propagation causes superimposition of the scattered light peaks, shown in FIG. 6, so the two cells are counted as one.
A possible solution is to reduce the size of the mask aperture, as shown in FIG. 9b. Although this arrangement decreases the probability of two cells being counted as one, it also decreases measurement volume which, when cell concentrations are low, degrades measurement reproducibility and, when visual observations are conducted results in inconsistent readings.